1. Field of the Invention
The present invention relates to a worm gear reducer which is used being incorporated into an electric power steering device, a nursing bed and so forth. The present invention also relates to a manufacturing method for a worm wheel composing the worm gear reducer.
2. Description of the Background Art
In order to reduce a force needed for operating a steering wheel when a driver gives a steering angle to vehicle wheels to be steered (Vehicle wheels to be steered are usually front wheels except for a special vehicle such as a forklift), a power steering device is generally used. As for this power steering device, an electric power steering device, in which an electric motor is used as an assisting power source, has recently spread. As compared with a hydraulic type power steering device, the electric type power steering device is advantageous as follows. The electric type power steering device can be made compact and further the weight can be reduced. Furthermore, an intensity of assisting torque can be easily controlled. Therefore, a loss of power caused in an engine can be decreased. In this case, a reduction gear is incorporated into this electric power steering device. Concerning this reduction gear, it is conventional to use a reversible type worm gear reducer having a large lead angle, the power transmitting direction of which can be reversed.
FIGS. 17 and 18 are views showing an example of the worm gear reducer of the background art into which the above electric type power steering device is incorporated. This worm gear reducer is provided in the reduction gear housing 1 which is fixed to the electric motor 1. The worm gear reducer includes: a worm shaft 4, in the intermediate portion in the axial direction of which the worm 3 is provided; and a worm wheel 5 meshed with this worm 3. The worm shaft 4 is pivotally supported inside the housing 2 by a pair of ball bearings 6, 6 which are outwardly engaged with both end portions in the axial direction of the worm shaft 4. When one end portion of the worm shaft 4, that is, the left end portion of the worm shaft 4 in FIG. 17, is connected with the output shaft 7 of the electric motor 1, this worm shaft 4 can be freely rotated being driven.
The worm wheel 5 is pivotally arranged inside the housing 2, and the rotary central axis of the worm wheel 5 itself is arranged at a twisting position with respect to the worm shaft 4. When the tooth portion 8 formed in the outer circumferential edge portion of the worm wheel 5 is meshed with the worm 3, torque can be freely transmitted between the worm wheel 5 and the worm shaft 4. In this connection, in the example shown in the drawing, the inner end portion and the intermediate portion in the radial direction of the worm wheel 5 are made of metal, and the outer end portion (the hatched portion shown in FIGS. 17 and 18) in the radial direction including the tooth portion 8 is made of synthetic resin 9. Due to the above structure, it is possible to reduce an intensity of the knocking and sliding noise generated in the meshing portion at the time of operation. This worm wheel 5 is outwardly fixed to an intermediate portion of the steering shaft 10. Due to this structure, it is possible to transmit a rotating drive force generated by the electric motor 1 to the steering shaft 10 via the worm gear reducer.
When the lead angle θ defined between the worm 3 and the worm wheel 5 is sufficiently large, a reverse operation of the worm gear reducer can be conducted. That is, a rotation can be transmitted from the worm wheel 5 to the worm 3. In this connection, in order to increase the lead angle θ between the worm 3 and the worm wheel 5 while the reduction ratio of the worm gear reducer and the outer diameter of the worm wheel 5 are being maintained constant, as shown in FIGS. 19A and 19B, it is necessary to decrease the outer diameter of the worm 3. Accordingly, in the worm gear reducer shown in FIGS. 17 and 18, as compared with the worm gear reducer in which the reversible operation not needed and the lead angle θ thereof is small, the outer diameter of the operational worm 3 is greatly decreased.
The electric type power steering device including the aforementioned worm gear reducer is operated as follows. When a driver operates a steering wheel so as to give a steering angle to the wheels to be steered, the steering shaft 10 is rotated. Then, a torque sensor (not shown) detects a rotary direction and torque of this steering shaft 10 and sends a signal expressing the thus detected value to a controller (not shown). This controller energizes the electric motor 1 and rotates the steering shaft 10 via the worm gear reducer in the same rotating direction as that of the operation of the steering wheel. As a result, the steering shaft 10 is rotated being given an assisting torque, which is generated by the electric motor 1 when it is energized, in addition to the torque generated by the operation of the steering wheel. Therefore, it is possible to decrease an intensity of torque, which is required for the driver to operate the steering wheel, by an intensity of torque generated by the assisting power. On the other hand, for example, when the electric motor 1 connected with the worm shaft 4 is out of order, the worm gear reducer can be reversed by operating the steering wheel. Due to the foregoing, it is possible to prevent the occurrence of a case in which the steering wheel can not be operated because of the problem of the electric motor 1.
In this connection, in the case where the worm wheel 5 having the above-described worm gear reducer is manufactured, it is common that the tooth portion 8 of this worm wheel 5 is machined with a hob cutter as described in Japanese Patent Unexamined Publication Nos. JP-A-2002-254478 and JP-A-2003-334724. FIG. 20 is a view showing circumstances in which the tooth portion 8 is machined with the hob cutter 11. In the case where this tooth portion 8 is machined, as shown in the drawing, the central axis of the intermediate material 12 (the worm wheel 5 in the state before the tooth portion is formed) of the worm wheel 5 and the central axis of the hob cutter 11 are arranged at positions so that the two central axes can be twisted from each other. In this state, the intermediate material 12 and the hob cutter 11 are respectively rotated at speeds according to the gear ratio of the worm gear reducer. Under this condition, the outer circumferential edge portion of the intermediate material 12 is cut with the hob cutter 11 to form the tooth portion 8 in the outer circumferential edge portion.
In the case where the tooth portion 8 is formed as described above, when the hob cutter 11 is used, the diameter of which is the same as that of the worm 3, it is possible to extend the contact area of the tooth portion 8, which has already been machined, with the worm 3, that is, the contact pressure can be decreased. Therefore, the durability of the worm gear reducer can be sufficiently ensured. However, as described before, in the case of the worm gear reducer incorporated into the electric type power steering device, it is necessary to make the electric type power steering device operate reversibly, that is, it is necessary to make the lead angle θ larger. Therefore, as compared with the worm gear reducer which is not reversibly operated, the lead angle θ of which is small, in the case of the worm gear reducer incorporated into the electric type power steering device, the outer diameter of the worm 3 is greatly decreased. Therefore, since the hob cutter 11 has pluralities of slits along with an axial direction thereof, when using the hob cutter 11 the diameter of which is the same as that of the worm 3, it becomes impossible to sufficiently ensure the mechanical strength of the hob cutter 11.
For the above reasons, the hob cutter 11, the diameter of which is larger than that of the worm 3, is actually used. However, only when the hob cutter 11 of large diameter is used as described above, the contact area of the tooth portion 8, which has already been machined, with the worm 3 (shown in FIGS. 17 to 19) is so small (it leads increase of contact pressure) that it becomes difficult to ensure the durability of the worm gear reducer. Therefore, in the actual case, after the tooth portion 8 has been machined with the hob cutter 11, the tooth portion 8 and the worm 3 are meshed with each other. Then, while a load is being given to this meshing portion, the worm gear reducer is driven being subjected to the running-in process. Due to the foregoing, the tooth face of the tooth portion 8 is machined, so that the contact area of the tooth portion 8 with the worm 3 can be increased. However, when the worm gear reducer is driven being subjected to running-in process as described above, the manufacturing cost of the worm gear reducer is raised.
Furthermore, recently, in order to enhance the efficiency of the worm gear reducer, the tooth face of the worm 3 is subjected to the mirror-finish process in many cases. In the case of the worm 3, the tooth face thereof is subjected to the mirror-finish process, even when this worm 3 is driven being subjected to the running-in process, it is impossible to cut the tooth face of the tooth portion 8. Accordingly, there is a possibility that an object of conducting the running-in process on the worm 3 can not be accomplished.